The industrial world depends a great deal on petroleum for energy. However, it has become increasingly clear that long term production cannot keep pace with the rapidly growing need, particularly in view of the growing demand from industrially developing countries.
Heavy oils represent by far the larger portion of the world's oil in place, yet represent only a minor portion of world oil production. With the normal yearly decrease in production from existing wells, production level can only be maintained by opening up new fields. Although the world is in no danger of soon running out of oil, it has become increasingly difficult to find new conventional oil fields. Thus, it is recognized that at some time in the not too distant future, production of conventional crude oils will peak and thereafter decrease regardless of continuing new discoveries. Thus, in the future, greatly increased production of heavy oils will be required.
Such heavy oil deposits can be recovered by mining and upgrading the recovered oil. However, by far the bulk of such heavy oil reserves occur at depths greater than that from which it can be recovered by known surface mining techniques. To overcome problems associated with such surface mining techniques, steam flooding extraction methods such as Steam Assisted Gravity Drainage (“SAGD”) have been developed. Steam flooding from surface steam generators is an effective and broadly applicable thermal recovery approach to enhanced oil recovery. The primary effects are reducing oil viscosity enough to allow flow and displacing the oil toward a production wellhead. The oil removed tends to be the more mobile fraction of the reservoir. However, in order to ensure compliance with national and local air pollution emission regulations, use of steam generators and the combustion emissions therefrom can limit their use, particularly in areas with more stringent emission regulations as in California.
Prior art steam flooding techniques face other limiting technical and economic obstacles relating to conductive heat losses through the wellbore and incomplete reservoir sweep efficiency, especially in heterogeneous reservoirs. This limits the depth from which oil can be recovered. In addition, steam boilers require relatively clean water to minimize fouling of heat transfer surfaces. Further, surface water is not always available. Without improved technology to deal with these issues, it is unlikely that heavy oil production can expand sufficiently to meet the growing demand for oil.
To overcome the wellbore heat loss problems involved in surface steam generation, there has been work on producing the steam downhole. Sandia Laboratories, under the U.S. Department of Energy (“DOE”) sponsorship, operated a downhole direct combustion steam generator (“Project Deepsteam”) burning natural gas and diesel at Long Beach, Calif., in the Wilmington field. Although there were initial problems relating to steam injectivity into the reservoir, results demonstrated the advantages in terms of reduced heat losses. However, the Project Deepsteam approach exhibited problems with soot formation in stoichiometric operation.
In a more advanced approach, in the 1980's Dresser Industries developed a catalytic downhole steam generator burning oil-water emulsions as described in U.S. Pat. Nos. 4,687,491 and 4,950,454. This approach eliminated soot formation and reduced heat loss in supplying steam to a formation, but it still required high purity water to avoid contaminate deposition on the catalyst. Moreover, heat output was limited by the need to vaporize the heavy oil used as fuel. Thus, these approaches have not been commercially employed.
Another problem associated with generating heat downhole is the lack of a robust method for the startup of the heat-generating operation. For example, spark igniters require exceedingly high voltage in applications exposed to high pressure. In another example, the use of a glow plug exposes the heat-generating operation to considerable downtime because of the glow plug's characteristically short life span.
With worldwide consumption of petroleum increasing year-by-year, there is a need to more efficiently produce oil from heavy crude oil deposits. Accordingly, there is need for a method of downhole heat generation which avoids the limitations of the prior art. More particularly, there is a need for a method of steam generation which reduces heat losses and does not rely on the availability of surface water, particularly if such method can utilize reservoir water without cleaning such water to boiler quality water. In addition, there is a need for such a method wherein ignition-on-contact is inherent.